ECHo physics

2.  Waves of sound are sent into the body and
‘bounce’ off structures
 The returning echoes are then collected &
used to produce an image

3. Sound waves are
produced within the
transducer
(Piezoelectric effect)
Sound propagates into tissue when the transducer is applied to the
patient. Coupling gel is required to combat air between interfaces.
Returning sound is captured by
the transducer, changed to
electrical signal and passed to
the processing unit for display
on the monitor.

4.  An electric current is briefly passed over the
layer of crystals present within the transducer
 The current physically alters the shape of the
crystal
 Once the current is switched off , the crystal
snaps back to it’s normal shape producing a
sound wave
 This cycle is repeated to produce continuous
scanning

5. Electrical charge
applied –
compression of
crystals
Electrical charge
off – rarefaction
of crystals
Normal crystal
Sound wave
emitted

6.  Refers to the resistance
offered by the tissue to
the travelling sound wave
 Determines how much
sound is transmitted
forward and how much is
reflected back
 Determined by the
density of the tissue (p)
and the speed of sound (c)
within that tissue type
Z= pc
Interface
between tissue
The amount of sound allowed to pass
through a border of two differing tissue
types will depend on how closely matched
the impedance values are – i.e. the greater
the mismatch the less sound throughput
(reflected sound)
(transmitted sound)
Original
sound
wave

7.  The more alike the neighbouring tissues are the more
sound is transmitted
 This is known as the intensity reflection co-efficient (R)
 Determines the ratio of energy reflected – where 0 =
complete transmission and 1 = complete reflection
Tissue borders Intensity reflection
coefficient (R)
Sound reflected - percentage
Soft tissue/ water 0.002 0.2
Fat/muscle 0.0108 1.08
Bone/muscle 0.412 41.2
Bone/fat 0.49 49.0
Air/soft tissue 0.999 99.9

8.  When a large amount
of sound is reflected
this produces a
brighter echo and
appears hyper echoic
on screen
 Trade off -there is very
little imaging
capability past this
strong reflector
 E.g. acoustic shadow
posterior to bone

9.  The strength of the sound
reflected back to the transducer
will also depend on the type of
interface it meets
 A smooth surface will return a
stronger echo (specular
reflection)
 An irregular surface will cause
the beam to scatter into more,
smaller echoes -therefore
reducing the overall strength
(diffuse reflection)

10.  The process by which a sound
beam will lose it’s intensity
 Reflection is one of the main
methods of attenuation
 A beam is attenuated on it’s way
in to the body but also on it’s
return path back to the
transducer
 Directly proportional to frequency
– i.e. higher frequencies
attenuate more therefore cannot
travel as far. Not generally a
problem for superficial MSK
scanning.
 Reflection
▪ Forming of echo
 Absorption
▪ Transformation of sound
energy into heat energy –
stored within tissue
 Scattering
▪ Form of reflection where the
majority of the echoes are not
returned to the transducer –
spread throughout the organ

11.  Refraction
▪ Change of beam pathway – occurs at
tissue borders where there is a mismatch
in speed of sound causing a change in
wavelength
 Divergence
▪ The widening of the beam from
origin – results in a loss of
intensity
Wider beam at the far
field than the near field
Tissue 1
Tissue 2
Snell’s law – formula that expresses the angles
of incidence (i) and refraction (r)
sinθi = c1
Sinθr c2
i
r

12.  Returning echoes are received by the
piezoelectric crystal within the transducer
 Crystal again is vibrated– this time converts
the vibration to an electrical signal
 The processing unit within the machine
recognises the position of each crystal – i.e.
this will allow the computer to plot the echo
on the screen depending on where on the
transducer it was received and the strength of
returning signal

13. The area of interest lies within the left field.As this is
detected by crystals towards the left of the transducer this
has been correctly plotted on the screen

14.  The system must additionally plot the correct
depth an echo has returned from
 The time taken for a returning echo to be
detected will determine how far it has
travelled
 E.g. a longer time will translate to a greater depth
displayed on screen

15. Time for pulse
and receive X
seconds
Time for pulse
and receive 2X
seconds

16.  The system plots every echo this way
 Bases it’s calculations on several assumptions
 Path of an echo is straight – original or returning
 The speed of sound (1540 m/s)and attenuation is
constant through all tissue
 All echoes have originated from the centre of the
beam
 Returning echoes – time taken for these to be
detected is directly proportional to the distance
travelled

17.  These assumptions can lead to false
appearances being displayed on the image
 These are known as artefacts
 Common artefacts include
 Acoustic shadow
 Acoustic enhancement
 Reverberation
 Edge shadow
 Mirror image

18.  Each echo is displayed as a
grey scale
 Level of grey determined
by the amplitude of the
echo received
 Strong echo = white
 No echo = black
 Approximately 64 grey
scales available on most
scanners – can alter
manually

19.  The ability of the
ultrasound machine to
distinguish different
structures and display
these correctly is
known as resolution
 Several types are
important in MSK
Ultrasound imaging:
 Spatial
▪ Axial
▪ Lateral
Spatial resolution – the ability to distinguish
two structures which lie very close together
Axial
Lateral

20.  Axial resolution
 Determined by pulse
length – i.e. short pulse
length = greater axial
resolution
 Affected by the
frequency – higher
frequencies have shorter
wavelengths and pulse
lengths
 Lateral resolution
 Determined by the beam
width – i.e. beam must
be narrow enough to
‘see between’ two
structures and register
them separately
 Affected by the use of
focus

21.  Axial  Lateral

22.  Temporal resolution
 Ability of the machine to
correctly distinguish two
separate actions occurring
at separate times and
display them accordingly –
i.e. the number of times a
section of tissue is scanned
and displayed
 Faster the update (frame
rate) = higher temporal
resolution – REALTIME
Sector width – the narrower the section the
faster the whole area can be imaged = more
times (increased frame rate) and better
temporal resolution
Focal zones – more focal zones require
longer sweeps and thus have lower
temporal resolutions
3 zones –
requires 3
sweeps
1 zone –
1 sweep

23.  Typical machine – important controls to increase image
quality/ detail TGC – used to alter amplitude of echoes at
different levels of the image – e.g. gradual
slope useful to even out echoes from deep
structures with those superficially. Differs
from overall gain which amplifies all echoes
equally
Also known as contrast – alters the
number of grey scales available for
the computer to use for display
Sector width – alters the number
of scan lines in an image
Frequency – alters the
resonant frequency employed
– higher for more superficial
structures = better detail
Depth – distance able to be
imaged

24.  Colour – demonstrates the
presence and direction of
flow. Dependent on incident
angle of beam.
 Spectral – allows calculations
of velocity and pulsatility to
be calculated. Dependent on
incident angle of beam.
 Power – demonstrates the
presence of flow; gives no
indication of direction. More
sensitive for low flow. Not
angle dependent and used
widely in MSK field.
colour
spectral
power

25.  Frequency of a sound wave is changed when it
encounters a moving object
 Increases when object moves closer to the wave
 Decreases when object moves further from the wave
 Proportional to the velocity of the moving target –
Doppler shift
Doppler shift equation

26.  As Low As Reasonably
Achievable
 Means use the least
amount of ultrasound to
gain a diagnostic image
 Overarching principle of
imaging – in ultrasound
this translates to
 Lowest output power
 Shortest scanning time
possible
 Use of supplementary
imaging techniques
(e.g. Doppler) increases
the output power.

27.  No evidenced adverse
effects of ultrasound in
human subjects
 Theoretical effects fall
into two categories
 Mechanical
▪ Stable cavitation
▪ Non-stable cavitation
 Thermal
 Cavitation –
 the creation of an empty
space within tissue

28.  Stable cavitation
 Tiny gas bubbles in
tissue
 Pressure of the sound
wave will cause a
transient change in
their size
 Considered relatively
safe
Compression
Rarefaction
Gas bubble
shrinks during
peak positive
pressure
Gas bubble
swells during
peak negative
pressure

29.  Unstable cavitation
 When there is high enough
intensity within the wave to
cause the gas bubbles to
burst and collapse
 Will cause damage to
surrounding material – i.e.
causes an ‘empty space’ to
appear where the gas
bubble once occupied
 Proven cases in rodent lung
High intensity
beam
Average intensity
beam
Normal
oscillation –
stable
cavitation
Gas bubble bursts
and destroys the
surrounding tissue
– creates a hole

30.  The heating of tissue
through which a sound
wave passes – process of
absorption
 Highest risk of heating
within tissue with high
absorption coefficient
e.g. bone
 Also affected by higher
frequencies and
screening times
Tissue Type Absorption Coefficient (dB/cm at 1mHz)
Bone 5
Muscle 1.3 – 3.3
Fat 0.63
Blood 0.18
Water 0.0022

31.  Ultrasound operators are responsible for
minimising the risk of bioeffects
 Manufacturers are required to display
information on the live image that will help
operators make informed decisions – safety
indices
 Thermal Index (TI)
 Mechanical Index (MI)

32. Determines the ratio of the power produced to
the power estimated to raise the temperature
of surrounding tissue by 1°C
TI =W
W deg
i.e. TI = overall power
power needed to raise
temp by 1°C

33. Estimates the potential for cavitation by the
formula below
MI = PNP
√f
i.e. the peak negative pressure divided by the
square root of operating frequency

34. MI Level Concern
MI > 0.3 Possible risk of slight damage
to the neonatal intestine/lung
tissue
MI > 0.7 Theoretical risk of cavitation.
Risk of cavitation in
conjunction with contrast
agent
TI Level Concern
TI > 1 Risk of increased tissue
temperature
TI > 3 Significant risk of increased
tissue temperature
The British
Medical
Ultrasound
Society (BMUS)
have previously
released
recommendations
on MI andTI
values
(Adapted from
Gibbs et al, 2010)

35.  TheTI and MI values can be controlled by
 Non- Stationary Probe – if specific tissue is
continually interrogated this can cause localised
heating.
 Frequency – higher frequency will cause increase
inTI due to higher absorption
 Focus – As the intensity is highest at the focus, the
site and number of focus points along a beam will
affect the MI/TI values

36.  British Medical Ultrasound Society (2009) Guidelines
for the safe use of diagnostic ultrasound equipment
https://www.bmus.org/static/uploads/resources/BMUS-Safety-Guidelines-2009-revision-FINAL-Nov-2009.pdf
 Gibbs,V., Cole, D. and Sassano, A. (2009). Ultrasound
physics and technology. Edinburgh: Churchill
Livingstone/Elsevier.
 Kremkau, F., Forsberg, F. and Kremkau, F. (2011).
Sonography principles and instruments. St. Louis, Mo.:
Elsevier/Saunders.